1. Field of the Invention
The present invention relates to a mist ejection head and an image forming apparatus, and more particularly, to a mist ejection head and an image forming apparatus which each eject mist.
2. Description of the Related Art
For example, each of Japanese Patent Application Publication No. 62-85948, Japanese Patent Application Publication No. 62-111757, Japanese Patent Application Publication No. 10-278253, and Japanese Patent Application Publication No. 2002-166541 discloses a mist ejection head which ejects a mist, in other words, a group (cluster) of minute liquid droplets.
Stated in simple terms, the ejection of mist is performed by creating a mist of liquid by reducing the surface tension of the liquid by means of an ultrasonic wave. More specifically, in general, cavitation misting caused by cavitation (hollowing), and capillary misting caused by a capillary wave (capillary surface wave), are used. If the latter type of method is used, then it is possible to generate a mist of uniform particle size, and the energy efficiency is good. A capillary wave is generated by applying a planar wave in the direction of the free liquid surface. If the planar wave has a frequency and amplitude at or above a certain level, then the capillary wave starts to oscillate. As a result, mist is produced from the peaks of the capillary wave thus developed.
Furthermore, each of Japanese Patent Application Publication No. 10-278253 and Japanese Patent Application Publication No. 2002-166541 discloses an apparatus for creating mist. As shown in FIG. 11, it is known that energy efficiency is improved by means of a method in which an ultrasonic wave which is generated by an ultrasonic wave generating element 958 and introduced into the liquid in the liquid chamber 952 is reflected by the paraboloid (parabolic surface) 980p of a reflector 980, and caused to concentrate at the focal point F located in the vicinity of the nozzle 51.
However, in the mist ejection head 950 shown in FIG. 11, the ultrasonic wave (direct advance wave) which has not yet been reflected by the paraboloid 980p has an effect on the ultrasonic wave (reflected wave) which has been reflected by the paraboloid 980p of the reflector 980, and hence there is a possibility that the energy efficiency declines. Furthermore, as the size of the reflector 980 becomes smaller with the increase in density, it becomes difficult to ignore the effects of the direct advance wave on the reflected wave, and hence the ultrasonic wave applied to the meniscus (liquid surface) of the nozzle 51 loses coherence, the particle size varies, and the ejection efficiency declines.
If described in detail with respect to FIG. 11, the ultrasonic wave introduced into the liquid in the liquid chamber 952 from the ultrasonic wave generating element 958 proceeds inside the liquid chamber 952 as a planar wave, and when it is reflected by the paraboloid 980p of the reflector 980, then the reflected ultrasonic wave (reflected wave) is concentrated at a focal point F situated in the vicinity of the nozzle 51. On the other hand, the ultrasonic wave (direct wave) which has not been reflected by the paraboloid 980p of the reflector 980 travels forward and arrives at the meniscus of the nozzle 51. A gap g (referred to as the spatial gap) occurs between the wave fronts of the reflected wave and the direct wave which are in the same phase, and this spatial gap g becomes larger as the ultrasonic wave progresses through the liquid chamber 952.
At any desired point Q on the paraboloid 980p, the distance QF from the focal point F is equal to the distance QD from the reference line D. Depending on this characteristic of the parabola, when the reflected wave reflected by the reflecting surface 980p reaches the focal point F, the distance between the wave fronts of the reflected wave and the direct wave which are in the same phase, is equal to the distance between the focal point F and the reference line D. Accordingly, unless a pinpoint design is adopted in which the interval between the focal point F and the reference line D is an integral multiple of the wavelength of the ultrasonic wave, then the coherence can be impaired and the energy efficiency can decline.